The present invention relates to a semiconductor device, and more particularly to a semiconductor device comprising a photovoltaic device having high open-circuit voltage under light irradiation or a photovoltaic device having improved photoelectric conversion efficiency.
Hitherto, as a material for photoelectric converters such as solar cells, there has been used a semiconductor material comprising an amorphous semiconductor such as a-Si:H, a-Si.sub.1-x :C.sub.x :H, a-Si.sub.1-x Ge.sub.x :H, a-Si:F:H, a-Si.sub.1-x N.sub.x H, a-Ge:H, a-Si.sub.1-x Ge.sub.x :F:H, a-Si:H, .mu.c-Si:H, .mu.c-Si.sub.1-x Ge.sub.x :H (wherein x satisfies the relationship 0 &lt;x &lt;1) or a semiconductor material composed partially of these amorphous semiconductors.
A conventional solar cell has a laminated structure pin, nip, pinpin. . . or nipnip. . . obtained by depositing in order the same kind of amorphous semiconductor or a different kind of amorphous semiconductor having a wide energy gap only in its doped layer. The density of dopant in the p-type or n-type layer is uniform throughout the direction of the thickness of the layer except for the distribution of dopant due to thermal diffusion during and after the production of the device, and it is usually 0.01 to 5 atm %.
FIG. 9 shows a semiconductor device employed in a conventional solar cell using a pin-type semiconductor of triple layer construction. In FIG. 9, numeral 31 is a glass substrate whereon a transparent electrode 32 is attached. On the transparent electrode 32, there are formed a p-type semiconductor layer 34 i-type semiconductor layer 35 and n-type semiconductor layer 36 in order. Further, on the n-type semiconductor layer 36, there is formed an electrode 37. The semiconductor device 38 consists of a glass substrate 31, a transparent electrode 32, a p-type semiconductor layer 34, an i-type semiconductor layer 35, an n-type semiconductor layer 36 and an electrode 37.
In the above semiconductor device 38, light enters the glass substrate 31 along the direction of the arrows in FIG. 9, is transmitted by the glass substrate 31 and the electrode 32, and then irradiates the p-type semiconductor layer 34, i-type semiconductor layer 35 and n-type semiconductor layer 36. By this irradiation, pairs of electrons and holes are generated in each semiconductor layer 34, 35 and 36. Then, electrons are collected in the n-type layer and holes are collected in the p-type layer and thereby, there are generated positive charges at the transparent electrode 32 and negative charges at the electrode 37. Thus, a photoelectric conversion is carried out enabling the semiconductor device 38 to function as a photocell.
However, a semiconductor device having the above construction has a drawback in that its voltage value in open-circuit condition (hereinafter referred to as Voc) during the irradiation of light cannot be increased, since the semiconductor device has a limitation in increasing a built-in field.
In using the above devices, for example, when an electromotive force greater than Voc is required, a plurality of devices can be connected in series in order to remove the above drawback. Even in that case, however, the number of series connections can be decreased if the Voc per device can be improved. Further, the performance of devices can be expected to be remarkably improved by making the area of each semiconductor layer large, even in case where the total area of all the devices is limited.
As a result of vigorous investigation focusing on the construction of a semiconductor, we inventors have discovered a semiconductor device having higher Voc and electric current (operating electric current) at a specific voltage than conventional semiconductor devices, without increasing the number of series connections and total area of all the devices, and have completed the semiconductor device of the present invention.
Furthermore, in the conventional semiconductor device, it is generally known that the contact resistance between a p-type semiconductor layer and the electrode at the side of the p-layer and between an n-type semiconductor layer and the electrode at the side of the n-layer decreases as the impurity density increases. It is desirable to make this contact resistance small, because it reduces the fill factor of a photovoltaic device when the p-type semiconductor layer and n-type semiconductor layer are used as a photovoltaic device such as a solar cell. Therefore, from this point of view, it is desirable to increase the impurity density. However, if the impurity density of the n-type and p-type semiconductor layers becomes too large, the characteristics of the photovoltaic device deteriorate due to the large absorption loss of light in the parts that contain the impurity.
The present invention was made to solve the above problems, and an object thereof is to provide a semiconductor device having high open-circuit voltage under light irradiation, and a semiconductor device having improved photoelectric conversion efficiency.